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Geotechnical Report
Turner Single Family Residence
413 N Hamma Ridge Dr, Lilliwaup
Parcel No. 32304-75-90091
Mason County, Washington
August 14, 2023
Project #23147
Prepared For:
Rebecca & Christian Turner
413 Bear Creek Estates
Sequim, WA 98382
Prepared By:
Envirotech Engineering
PO Box 984
Belfair, Washington 98528
Phone: 360-275-9374
08/14/2023
Instructions:
This checklist must be submitted with a Geotechnical Report and completed, signed, and stamped by the licensed
professional(s) who prepared the Geotechnical Report for review by Mason County pursuant to the Mason County
Resource Ordinance. If an item is found not applicable, the report should explain the basis for the conclusion.
Note: Unless specifically documented, this report does not provide compliance to the International Residential Code
Sections R403.1.7 for foundations on or adjacent to slopes, Section R403.1.8 for expansive soils or section 1808.7.1
of the International Building Code Section for Foundations on or adjacent to slopes.
Applicant/Owner Rebecca & Christian Turner Parcel # 32304-75-90091
Site Address 413 N Hamma Ridge Dr
(1) (a) A discussion of general geologic conditions in the vicinity of the proposed development,
Located on page(s) 5
(b) A discussion of specific soil types,
Located on page(s) 6
(c) A discussion of ground water conditions,
Located on page(s) 7
(d) A discussion of the upslope geomorphology,
Located on page(s) 3
(e) A discussion of the location of upland waterbodies and wetlands,
Located on page(s) 3
(f) A discussion of history of landslide activity in the vicinity, as available in the referenced maps and
records.
Located on page(s) 8
(2) A site plan which identifies the important development and geologic features.
Located on Map(s) Site Plan – Appendix A
(3) Locations and logs of exploratory holes or probes.
Located on Map(s) Site Plan and Soil Logs (Appendix B)
(4) The area of the proposed development, the boundaries of the hazard, and associated buffers and
setbacks shall be delineated (top, both sides, and toe) on a geologic map of the site.
Located on Map(s) Site Plan
(5) A minimum of one cross section at a scale which adequately depicts the subsurface profile, and
which incorporates the details of proposed grade changes.
Located on Map(s) Soil Profile (Appendix B)
(6) A description and results of slope stability analyses performed for both static and seismic loading
conditions. Analysis should examine worst case failures. The analysis should include the Simplified
Bishop’s Method of Circles. The minimum static safety factor is 1.5, the minimum seismic safety
factor is 1.1, and the quasi-static analysis coefficients should be a value of 0.15.
Located on page(s) 9 Page 1 of 41
(7) (a) Appropriate restrictions on placement of drainage features,
Located on page(s) 18
(b) Appropriate restrictions on placement of septic drain fields,
Located on page(s) 17
(c) Appropriate restrictions on placement of compacted fills and footings,
Located on page(s) 15
(d) Recommended buffers from the landslide hazard areas shoreline bluffs and the tops of other slopes.
Located on page(s) 18
(e) Recommended setbacks from the landslide hazard areas shoreline bluffs and the tops of other slopes.
Located on page(s) 17
(8) Recommendations for the preparation of a detailed clearing and grading plan which specifically identifies
vegetation to be removed, a schedule for vegetation removal and replanting, and the method of vegetation
removal.
Located on page(s) 18
(9) Recommendations for the preparation of a detailed temporary erosion control plan which identifies the specific
mitigating measures to be implemented during construction to protect the slope from erosion, landslides and
harmful construction methods.
Located on page(s) 10
(10) An analysis of both on-site and off-site impacts of the proposed development.
Located on page(s) 12
(11) Specifications of final development conditions such as, vegetative management, drainage, erosion control, and
buffer widths.
Located on page(s) 18
(12) Recommendations for the preparation of structural mitigation or details of other proposed mitigation.
Located on page(s) 19
(13) A site map drawn to scale showing the property boundaries, scale, north arrow, and the location and nature
of existing and proposed development on the site.
Located on Map(s) Site Plan
I, Michael Staten, hereby certify under penalty of perjury that I am a civil engineer licensed in the State of Washington
with specialized knowledge of geotechnical/geological engineering or a geologist or engineering geologist licensed in
the State of Washington with special knowledge of the local conditions. I also certify that the Geotechnical
Disclaimer: Mason County does not
certify the quality of the work done in
this Geotechnical Report.
Report, dated August 14, 2023 and entitled Turner Single
Family Residence, meets all the requirements of the Mason
County Resource Ordinance, Geologically Hazardous Areas
Section, is complete and true, that the assessment
demonstrates conclusively that the risks posed by the landslide
hazard can be mitigated through the included geotechnical
design recommendations, and that all hazards are mitigated in
such a manner as to prevent harm to property and public
health and safety.
Page 2 of 2
08/14/2023
TABLE OF CONTENTS
1.0 INTRODUCTION ................................................................................................................................. 1
1.1 PROJECT INFORMATION .................................................................................................................... 1
1.2 PURPOSE OF INVESTIGATION AND SCOPE OF WORK ........................................................................ 1
2.0 SURFACE CONDITIONS.................................................................................................................... 3
2.1 GENERAL OBSERVATIONS.................................................................................................................. 3
2.2 TOPOGRAPHY ..................................................................................................................................... 3
2.2.1 Upslope Geomorphology ............................................................................................................ 3
2.3 SURFACE DRAINAGE .......................................................................................................................... 3
2.3.1 Upslope Water Bodies ................................................................................................................ 3
2.4 SLOPE AND EROSION OBSERVATIONS ............................................................................................... 4
3.0 SUBSURFACE INVESTIGATION ..................................................................................................... 5
3.1 FIELD METHODS, SAMPLING AND FIELD TESTING ........................................................................... 5
3.2 GENERAL GEOLOGIC CONDITIONS ................................................................................................... 5
3.3 SPECIFIC SUBSURFACE CONDITIONS ................................................................................................. 6
3.3.1 Groundwater............................................................................................................................... 7
4.0 ENGINEERING ANALYSES AND CONCLUSIONS ...................................................................... 8
4.1 SLOPE STABILITY ............................................................................................................................... 8
4.1.1 Slope Stability Analysis .............................................................................................................. 9
4.2 EROSION ............................................................................................................................................10
4.3 SEISMIC CONSIDERATIONS AND LIQUEFACTION..............................................................................10
4.3.1 Liquefaction ..............................................................................................................................11
4.4 LANDSLIDE, EROSION AND SEISMIC HAZARDS CONCLUSIONS ........................................................11
4.5 LATERAL EARTH PRESSURES ...........................................................................................................11
4.6 ON-SITE AND OFF-SITE IMPACTS .....................................................................................................12
5.1 BUILDING FOUNDATION RECOMMENDATIONS.................................................................................13
5.1.1 Bearing Capacity .......................................................................................................................13
5.1.2 Settlement ..................................................................................................................................14
5.1.3 Concrete Slabs-on-Grade ..........................................................................................................14
5.2 EARTHWORK CONSTRUCTION RECOMMENDATIONS .......................................................................14
5.2.1 Excavation .................................................................................................................................14
5.2.2 Placement and Compaction of Native Soils and Engineered Fill ...........................................15
5.2.3 Retaining Wall Backfill .............................................................................................................16
5.2.4 Wet Weather Considerations ....................................................................................................16
5.2.5 Building Pads ............................................................................................................................16
5.3 BUILDING AND FOOTING SETBACKS .................................................................................................17
5.4 SURFACE AND SUBSURFACE DRAINAGE ...........................................................................................17
5.5 VEGETATION BUFFER AND CONSIDERATIONS .................................................................................18
5.6 TEMPORARY AND PERMANENT EROSION CONTROL .......................................................................18
5.7 SEPTIC DRAINFIELDS ........................................................................................................................19
5.8 STRUCTURAL MITIGATION ...............................................................................................................19
6.0 CLOSURE .............................................................................................................................................20
Appendix A - Site Plan
Appendix B - Soil Information
Appendix C - Slope Stability
Appendix D – Erosion Control
Appendix E – Drainage Details
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Envirotech Engineering Geotechnical Report
PO Box 984 page 1 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
1.0 INTRODUCTION
Envirotech Engineering (Envirotech) has completed a geotechnical investigation for a planned
single family residence located at 413 N Hamma Ridge Dr, identified as parcel number 32304-75-
90091, Mason County, Washington. See the vicinity map on the following page for a general
depiction of the site location.
An initial geotechnical evaluation of the project was conducted by Envirotech on May 12, 2023. It
was determined that slopes in excess of 40% with a vertical relief of at least 10 feet were present
within 300 feet of the planned development. Based on this site characteristic, the proposed
development will require a geotechnical report pursuant to Landslide Hazard Areas of Mason
County Resource Ordinance (MCRO) 8.52.140. During the site visit by Envirotech, surface and
subsurface conditions were assessed. After completion of the field work and applicable project
research, Envirotech prepared this geotechnical report which, at a minimum, conforms to the
applicable MCRO.
As presented herein, this report includes information pertaining to the project in this Introduction
Section; observations of the property and surrounding terrain in the Surface Conditions Section;
field methods and soil descriptions in the Subsurface Investigation Section; supporting
documentation with relation to slope stability, erosion, seismic considerations, and lateral earth
pressures in the Engineering Analyses and Conclusions Section; and, recommendations for
foundation, settlement, earthwork construction, retaining walls, erosion control, drainage, and
vegetation in the Engineering Recommendations Section.
1.1 Project Information
Information pertaining to the planned development of the project was provided by the proponent
of the property. The planned development consists of a 1- or 2-story single family residence, new
on-site septic system, and other ancillary features typical of this type of development. Approximate
building footprint and other proposed features with relation to existing site conditions are illustrated
on the Site Map provided in Appendix A of this report.
1.2 Purpose of Investigation and Scope of Work
The purpose of this geotechnical investigation is to assess geological hazards, and evaluate the
project in order to provide geotechnical recommendations that should be implemented during
development. The investigation included characterizing the general project surface and subsurface
conditions, and evaluating the suitability of the soils to support the planned site activities.
In order to fulfill the purpose of investigation, the geotechnical program completed for the proposed
improvements of the project include:
• Review project information provided by the project owner and/ or owner‘s representative;
• Conduct a site visit to document the site conditions that may influence the construction and
performance of the proposed improvements of the project;
• Define general subsurface conditions of the site by observing subsoils within test pits and/
or cut banks, review geological maps for the general area, research published references
concerning slope stability, and review water well reports from existing wells near the
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Envirotech Engineering Geotechnical Report
PO Box 984 page 2 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
project;
• Collect bulk samples, as applicable, at various depths and locations;
• Perform soils testing to determine selected index and/or engineering properties of the site
soils;
• Complete an engineering analysis supported by the planned site alterations, and the surface
and subsurface conditions that were identified by the field investigation, soil testing, and
applicable project research; and,
• Establish conclusions based on findings, and make recommendations for foundations,
drainage, slope stability, erosion control, earthwork construction requirements, and other
considerations.
Vicinity Map from Mason County
Project
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Envirotech Engineering Geotechnical Report
PO Box 984 page 3 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
2.0 SURFACE CONDITIONS
Information pertaining to the existing surface conditions for the project was gathered on May 12,
2023 by a representative with Envirotech. During the site visit, the type of geotechnical
investigation was assessed, site features were documented that may influence construction, and site
features were examined that may be influenced by construction. This Surface Conditions Section
provides information on general observations, vegetation, topography, drainage and observed
slope/ erosion conditions for the project and surrounding areas that may impact the project.
2.1 General Observations
Currently, the property is vacant. Vegetation on and near the project consists primarily of secondary
growth firs, alders, cedars and other trees and shrubbery common to this area of the Pacific
Northwest. An aerial photo of the project and immediate vicinity is provided on the following page.
2.2 Topography
The topographic information provided in this section was extrapolated from a public lidar source,
and incorporated observations and field measurements. Where necessary, slope verification
included measuring slope lengths and inclinations with a cloth tape and inclinometer. See the Site
Plan in Appendix A in this report for an illustration of general topography with respect to the
planned development.
Critical descending slopes, with grades exceeding 40% appear to be within 300 feet of the planned
development. The maximum critical slope is approximately 78% with a vertical relief of about 200
feet.
Critical ascending slopes, with grades exceeding 40% appear to be within 300 feet of the planned
development. The maximum critical slope is approximately 71% with a vertical relief of about 20
feet.
2.2.1 Upslope Geomorphology
The upland area of the property and beyond is generally situated on a hillside of glacial
origin.
2.3 Surface Drainage
Runoff originating upslope of the development is mostly diverted away from the property by
accommodating topography. Excessive scour, erosion or other indications of past drainage
problems were not observed within the immediate vicinity of the planned development.
2.3.1 Upslope Water Bodies
There are no apparent water bodies or wetlands located upslope from the planned
development that would significantly influence the project.
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Envirotech Engineering Geotechnical Report
PO Box 984 page 4 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
2.4 Slope and Erosion Observations
The slope gradients near the project signal a potential landslide or erosion hazard area. Some
indicators that may suggest past slope movements include:
• Outwash of sediments near the bottom of the slope,
• Fissures, tension cracks, hummocky ground or stepped land masses on the face or top of
the slope, and parallel to the slope,
• Fine, saturated subsurface soils,
• Old landslide debris,
• Significant bowing or leaning trees, or,
• Slope sloughing or calving.
These slope instability indicators or other significant mass wasting on the property or within the
general vicinity of the project were not observed. Indications of past landslides, current unstable
slopes, deep-seated slope problems, or surficial slope failures were not observed during the site
visit.
Aerial Photo from Mason County
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Envirotech Engineering Geotechnical Report
PO Box 984 page 5 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
3.0 SUBSURFACE INVESTIGATION
Information on subsurface conditions pertaining to the project was primarily gathered on May 12,
2023 by a representative with Envirotech. Applicable information on field methods, sampling, field
testing, general geologic conditions, specific subsurface conditions, and results from soil testing
are presented in this section of the report. Appendix B of this report includes pertinent information
on subsurface conditions for the project, such as subsoil cross-section(s), test pit log(s), and
applicable water well report(s). Water well reports were utilized to estimate ground water levels,
and if sufficient, were used in identifying subsoil types. Applicable test pit locations are depicted
on the Site Plan provided in the appendix of this report.
3.1 Field Methods, Sampling and Field Testing
Information on subsurface conditions for the project was accomplished by examining soils within
test pits and/ or nearby banks extending to depths of up to 8 feet below the natural ground surface.
Information on subsurface conditions also included reviewing geological maps representing the
general vicinity of the project, and water well reports originating from nearby properties.
Soil samples were not obtained from this project. Envirotech measured the relative density of the
near-surface in-situ soils by gauging the resistance of hand tools. Within testing locations, field
testing results generally indicated loose to medium dense soils in the upper 48 inches, and dense
soils from 48 inches to the depth of terminus.
3.2 General Geologic Conditions
In general, soils at the project are composed of materials from glacial advances. The geologic
conditions as presented in the “Geologic Map of Washington,” compiled by J. Eric Schuster, 2002
indicates Quaternary sediments, Qg. Quaternary sediments are generally unconsolidated deposits,
and dominantly deposited from glacial drift, including alluvium deposits. This project is located
within the Puget Lowland. Typically, “lower tertiary sedimentary rocks unconformably overlie the
Crescent Formation.” as revealed in the Geologic Map. Initial sedimentary rocks were formed from
shales, sandstones and coal deposits from rivers. During the Quaternary period, the Puget Lowland
was covered by numerous ice sheets, with the most recent being the Fraser glacier with a peak of
approximately 14,000 years ago. Upon the glacial retreat, the landscape was formed by glacial
erosion glacial drift deposits.
The "Geologic Map of the Eldon 7.5-Minute Quadrangle, Jefferson, Kitsap, and Mason Counties,
Washington" by Trevor Contreras, et al, 2012, provides the following caption(s) for the project
area:
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Envirotech Engineering Geotechnical Report
PO Box 984 page 6 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
Geological Map Department of Natural Resources Washington State
3.3 Specific Subsurface Conditions
The following subsurface conditions are estimated descriptions of the project subgrade utilizing
information from the depth of penetration at all testing, sampling, observed and investigated
locations. Soils for this project were primarily described utilizing the Unified Soil Classification
System (USCS) and the Soil Conservation Service (SCS) descriptions.
The project is currently composed of native soils without indications of fill where development is
expected. Within test pit locations, soils within the upper 8 feet of natural ground were generally
observed to be moist, brown silty sand with gravel (SM).
The relative densities of the soil within selected test pits are provided above in Section 3.1.
Expanded and specific subsurface descriptions, other than what is provided in this section, are
provided in the soil logs located in Appendix B of this report.
According to the “Soil Survey of Mason County,” by the United States Department of Agriculture,
Soil Conservation Service, the site soils are described as Hoodsport stony sandy loam, Hh, with
15% to 30% slopes, and Hoodsport gravelly sandy loam, Hf, with 30% to 45% slopes. The soil
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Envirotech Engineering Geotechnical Report
PO Box 984 page 7 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
designations are depicted in the aerial photograph below, and descriptions are provided in
Appendix B of this report.
Soil Survey from USDA Natural Resources Conservation Service
3.3.1 Groundwater
From the water well report(s) and knowledge of the general area, permanent groundwater
is at least 100 feet directly below the property at the building pad location. Surface seepage
or perched groundwater at shallow depths was not observed on-site, nor indicated on the
well reports.
_____________________________________________________________________________________
Envirotech Engineering Geotechnical Report
PO Box 984 page 8 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
4.0 ENGINEERING ANALYSES AND CONCLUSIONS
The following section includes slope stability, erosion, seismic considerations, and impacts to both
on-site and off-site properties.
4.1 Slope Stability
Landslides are natural geologic processes, and structures near slopes possess an inherent risk of
adverse settlement, sliding or structural damage due to these processes. Geotechnical engineering
cannot eliminate these risks for any site with sloping grades because gravity is constantly inducing
strain on the sloping soil mass. Excessive wet weather and/ or earthquakes will exacerbate these
strains. Geotechnical engineering considers excessive wet weather and ‘design’ earthquakes in
order to provide an acceptable factor of safety for developing on or near sloping terrain with relation
to current engineering protocol. These factors of safeties are based on engineering standards such
as defining engineering properties of the soil, topography, water conditions, seismic acceleration
and surcharges. Surface sloughing or other types of surficial slope movements usually do not affect
the deep-seated structural capability of the slope. However, repeated surficial slope movements, if
not repaired, may present a threat to the structural integrity of the slope. If any slope movement
arises, the slope should be inspected by an engineer. Subsequently, maintenance may be required
in order to prevent the possibility of further surficial or deep seated slope movements that may be
damaging to life and property.
According to the Coastal Zone Atlas of Mason County, Washington, the project is within and near
terrain labeled ‘Unstable’ and ‘Intermediate’ regarding potential landslide activity. Descriptions of
these mapping units may be found in the aforesaid Atlas. A Stability Map from the Coastal Zone
Atlas for the general area of this project is provided below:
Map from Washington State Department of Ecology
Project
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Envirotech Engineering Geotechnical Report
PO Box 984 page 9 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
According to the Resource Map from the Washington State Department of Natural Resources
(DNR), the project is within terrain labeled ‘highly unstable’ relating to soils. DNR labeled portions
of this project as medium and high slope instability with relation to slopes. A Resource Map from
the DNR Forest Practices Application Review System is provided below:
Resource Map from Washington State Department of Natural Resources
4.1.1 Slope Stability Analysis
The Simplified Bishop Method, utilizing ‘STABLE’ software, was used to analyze the
static stability of the site slopes. Seismic conditions were estimated utilizing worst case
scenario values from the static analysis, a quasi-static analysis coefficient of at least 0.15,
and applying the applicable values to STABLE software. Various radii and center points
of the circle were automatically selected, and produced factor of safeties in a graphical and
tabular format. Worst case scenario values were used in the slope stability analysis in
regard to topography, surcharges, water content, internal friction and cohesion of the site
soils. STABLE software has been repeatedly checked with manual calculations, and
consistently proved to be a very conservative program. The following soil properties were
used in the analysis, and are based on observed conditions, known geology, and/or
published parameters:
Soil unit weight: 132 pcf
Angle of internal friction: 30 degrees
Cohesion: 200 psf
Based on the slope stability analysis, unacceptable factors of safety could be present on
and near the critical slope, but do not reflect conditions where development is expected to
Project
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Envirotech Engineering Geotechnical Report
PO Box 984 page 10 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
occur. For this project, at the location of the proposed development, minimum factor of
safeties for static and dynamic conditions were estimated to be at least 1.5 and 1.1,
respectively. See the slope stability information in Appendix C for a depiction of minimum
factors of safety away from the project.
4.2 Erosion
Based on the USCS description of the project soils, the surface soils are considered moderately
erodible. According to the Resource Map from the Washington State DNR, as provided above, the
project is not within terrain labeled ‘highly erodible.’ This project is not within an erosion hazard
area as defined by the MCRO. Erosion hazard areas are those with USDA SCS designations of
River Wash (Ra), Coastal Beaches (Cg), Alderwood Gravelly Sandy Loam on slopes 15% or
greater (Ac and Ad), Cloquallum Silt Loam on slopes 15% or greater (Cd), Harstine Gravelly Sandy
Loam on slopes 15% or greater (Hb), and Kitsap Silt Loam on slopes 15% or greater (Kc).
It is our opinion that minor erosion control recommendations provided in this report is sufficient
for the development of this project, and additional engineered erosion control plans are not
required. Temporary and permanent erosion control measures are required for site development.
Extents of temporary erosion control will mostly depend on the timeliness of construction, moisture
content of the soil, and amount of rainfall during construction. Soil erosion typical to the existing
site conditions and planned disturbance of the project include wind-borne silts during dry weather,
and sediment transport during prolonged wet weather. Sediment transport could be from
stormwater runoff or tracking off-site with construction equipment.
The Temporary and Permanent Erosion Control Section (Section 5.6) of this report consist of
specific erosion controls to be implemented. Additional erosion control information and
specifications may be found in the latest addition of the “Stormwater Management Manual for
Western Washington,” prepared by the Washington State Department of Ecology Water Quality
Program.
4.3 Seismic Considerations and Liquefaction
There are no known faults beneath this project. The nearest Class ‘A’ or Class ‘B’ fault to this
property is the Hood Canal fault zone (class B), which is approximately 1 miles to the south east
of this project. This information is based on the USGS Quaternary Fault and Fold Database for the
United States.
Potential landslides due to seismic hazards have been considered, and are addressed in the Slope
Stability Analysis Section provided earlier in this report.
Soils immediately below the expected foundation depth for this project are generally Type D,
corresponding to the International Building Code (IBC) soil profiles. According to the IBC, the
regional seismic zone is 3 for this project. The estimated peak ground acceleration ranges from
0.50g to 0.60g. This estimation is based on the United States Geological Survey (USGS) National
Seismic Hazard project in which there is an estimated 2% probability of exceedance within the next
50 years.
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Envirotech Engineering Geotechnical Report
PO Box 984 page 11 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
4.3.1 Liquefaction
The potential for liquefaction is believed to be low for this project. This is based, in part,
on the subsurface conditions such as soil characteristics and the lack of a permanent
shallow water table. Subgrade characteristics that particularly contribute to problems
caused from liquefaction include submerged, confined, poorly-graded granular soils (i.e.
gravel, sand, silt). Although gravel- and silt-sized soil particles could be problematic, fine
and medium grained sands are typically subjected to these types of seismic hazards. No
significant saturated sand stratifications are anticipated to be within the upper 50 feet of
the subsoil for this project.
4.4 Landslide, Erosion and Seismic Hazards Conclusions
DNR did not indicate historic landslide activity near the project. Mapped slope conditions, as
delineated by the Departments of Ecology and/ or Natural Resources, were considered in our slope
stability assessment. Based on the proximity and severity of mapped delineations with respect to
the proposed development, results of the aforesaid slope stability analysis, observed surface
conditions, and other pertinent information, it is our opinion that the proposed development may
occur in accordance with the recommendations in this geotechnical report.
4.5 Lateral Earth Pressures
Retaining walls may be utilized for this Project. The lateral earth pressures exerted through the
backfill of a retaining wall are dependent upon several factors including height of retained soil
behind the wall, type of soil that is retained, degree of backfill compaction, slope of backfill,
surcharges, hydrostatic pressures, earthquake pressures, and the direction and distance that the top
of the wall moves.
An equivalent fluid unit weight used for structural design may be estimated as the product of the
backfill soil unit weight and the earth pressure coefficient for at-rest pressures. Retaining walls
should be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of the
following:
At-Rest Active
Native Soils 49 pcf 32 pcf
Engineered Fill Soils 45 pcf 28 pcf
The values provided above shall be increased by 1 pcf for every 1 degree of backfill/ natural slope
angle. These equivalent fluid unit weight values do not include lateral earth pressures induced by
earthquakes, groundwater, or surcharges from live loads. Earthquake pressures should be added to
the wall analysis, and treated as an inverted pressure triangle where the resultant pressure is located
at 2/3 of the wall height, or other method approved by a structural engineer. The following resultant
earthquake pressures as a function of the wall height (H) may be utilized:
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Envirotech Engineering Geotechnical Report
PO Box 984 page 12 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
At-Rest Active
Native Soils 15.4H psf 9.8H psf
Engineered Fill Soils 13.6H psf 8.2H psf
See the Earthwork Construction Recommendations Section for details concerning the use of native
soils, engineered fill and placement of backfill.
4.6 On-Site and Off-Site Impacts
From a geotechnical position, it is Envirotech’s opinion that the subject property and adjacent
properties to the proposed development should not be significantly impacted if all
recommendations in this report are followed. This opinion is based on the expected site
development, existing topography, existing nearby development, land cover, and adhering to the
recommendations presented in this report. Future development or land disturbing activities on
neighboring properties or properties beyond adjacent parcels that are upslope and/or downslope
from the subject property could cause problems to the subject property. For this reason, future
development or land disturbance near the subject property should be evaluated by a geotechnical
engineer.
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Envirotech Engineering Geotechnical Report
PO Box 984 page 13 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
5.0 ENGINEERING RECOMMENDATIONS
The following sections present engineering recommendations for the proposed improvements of
the project. These recommendations have been made available based on the planned improvements
as outlined in the Introduction Section of this report; general observations including drainage and
topography as recapitulated in the Surface Conditions Section; soil/ geologic conditions that were
identified from the geotechnical investigation that is summarized in the Subsurface Investigation
Section; and, project research, analyses and conclusions as determined in the Engineering Analysis
and Conclusions Section. Recommendations for the project that is provided herein, includes
pertinent information for building foundations, earthwork construction, building and/ or footing
setbacks, drainage, vegetation considerations, and erosion control.
5.1 Building Foundation Recommendations
Recommendations provided in this section account for the site development of a typical one- or
two-story, single family residential structure. The recommended allowable bearing capacities and
settlements as presented below, consider the probable type of construction as well as the field
investigation results by implementing practical engineering judgment within published engineering
standards. Evaluations include classifying site soils based on observed field conditions and soil
testing for this project. After deriving conservative relative densities, unit weights and angles of
internal friction of the in-situ soils, the Terzhagi ultimate bearing capacity equation was utilized for
determining foundation width and depth. Foundation parameters provided herein account for
typical structural pressures due to the planned type of development. A structural analysis is beyond
the scope of a geotechnical report, and a structural engineer may be required to design specific
foundations and other structural elements based on the soil investigation. Stepped foundations are
acceptable, if warranted for this project. Continuous, isolated, or stepped foundations shall be
horizontally level between the bottom of the foundation and the top of the bearing strata. The frost
penetration depth is not expected to extend beyond 12 inches below the ground surface for this
project under normal circumstances and anticipated design features.
5.1.1 Bearing Capacity
Existing in-situ soils for this project indicates that the structure can be established on
shallow, continuous or isolated footings. Foundations shall be established on relatively
undisturbed native soil that is competent and unyielding. Alternatively, foundations may
be constructed on selective re-compacted native soil or compacted engineered fill as
described in the Earthwork Construction Recommendations Section of this report.
For a bearing capacity requirement of no more than 1500 psf, a minimum continuous
footing width of 15 inches shall be placed at a minimum of 18 inches below the existing
ground surface atop unyielding soils. Where slopes are greater than 20%, foundations shall
be extended to at least 2.5 feet below the ground surface as measured from the downslope
side of the footing. For a columnar load of no more than 2.5 tons, a circular or square
isolated foundation diameter or width shall be at least 24 inches. Foundation
recommendations are made available based on adherence to the remaining
recommendations that are provided in this report. Alterations to the aforementioned
foundation recommendations may be completed upon a site inspection by a geotechnical
engineer after the foundation excavation is completed.
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Envirotech Engineering Geotechnical Report
PO Box 984 page 14 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
5.1.2 Settlement
Total and differential settlement that a structure will undergo depends primarily on the
subsurface conditions, type of structure, amount and duration of pressure exerted by the
structure, reduction of pore water pressure, and in some instances, the infiltration of free
moisture. Based on the expected native soil conditions, anticipated development, and
construction abides by the recommendations in this report, the assumed foundation system
may undergo a maximum of 1.0 inch total settlement, and a maximum differential
settlement of 0.75 inch.
5.1.3 Concrete Slabs-on-Grade
Interior slabs, if utilized, should be supported on a minimum of 4 inches of compacted
coarse, granular material (Retained on U.S. Sieve #10 or greater) that is placed over
undisturbed, competent native subgrade or engineered fill per the Earthwork
Recommendations Section below.
The recommendations for interior concrete slabs-on-grade as presented herein are only
relevant for the geotechnical application of this project. Although beyond the scope of this
report, concrete slabs should also be designed for structural integrity and environmental
reliability. This includes vapor barriers or moisture control for mitigating excessive
moisture in the building.
5.2 Earthwork Construction Recommendations
Founding material for building foundations shall consist of undisturbed native soils to the specified
foundation depths. Compacted engineered fill, or selective re-compacted native soils may be used
to the extents provided in this Earthwork Construction Recommendations Section. The following
recommendations include excavations, subgrade preparation, type of fill, and placement of fill for
building foundations.
5.2.1 Excavation
Excavation is recommended to remove any excessive organic content or other deleterious
material, if present, beneath foundations and to achieve appropriate foundation depth.
Additional sub-excavation will be required for this project if the soils below the required
foundation depth are loose, saturated, not as described in this report, or otherwise
incompetent due to inappropriate land disturbing, or excessive water trapped within
foundation excavations prior to foundation construction. All soils below the bottom of the
excavation shall be competent, and relatively undisturbed or properly compacted fill. If
these soils are disturbed or deemed incompetent, re-compaction of these soils below the
anticipated footing depth is necessary. Excavations shall be completely dewatered,
compacted, and suitable before placement of additional native soil, engineered fill or
structural concrete.
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Envirotech Engineering Geotechnical Report
PO Box 984 page 15 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
5.2.2 Placement and Compaction of Native Soils and Engineered Fill
For engineered fill or disturbed native soils that will be utilized as fill material directly
beneath foundations, observation and/ or geotechnical testing is required prior to
foundation construction. The following placement and compaction requirements are
necessary.
For disturbed native soils or engineered fill beneath foundations, limits of compacted or
re-compacted fill shall extend laterally from the bottom edge of the foundation at a rate of
one horizontal foot for each foot of compacted or re-compacted fill depth beneath the
foundation. See the illustration below.
Both engineered fill and native soils used as compacted fill should be free of roots and
other organics, rocks over 6 inches in size, or any other deleterious matter. Because of
moisture sensitivity, importing and compacting engineered fill may be more economical
than compacting disturbed native soils. Engineered fill shall include having the soils
retained on the No. 4 sieve crushed (angular), and should consist of the following
gradation:
U.S. Standard Sieve % Finer (by weight)
6” 100
3” 60 – 100
No. 4 20 – 60
No. 200 0 - 8
Table 1
Particle Size Distribution of Engineered Fill
Compaction shall be achieved in compacted lifts not to exceed 6 inches for both native
soils and engineered fill, respectively. Each lift should be uniformly compacted to at least
95% of the modified Proctor maximum dry density (ASTM D 1557) and within 3% of
optimum moisture content. Each lift surface should be adequately maintained during
construction in order to achieve acceptable compaction and inter-lift bonding.
Temporary earth cuts and temporary fill slopes exceeding 4 feet in height should be limited
to a slope of 2:1 (horizontal:vertical). Utility trenches or other confined excavations
exceeding 4 feet should conform to OSHA safety regulations. Permanent cut and fill slopes
shall be limited to a slope of 2:1, unless otherwise approved by an engineer.
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Envirotech Engineering Geotechnical Report
PO Box 984 page 16 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
5.2.3 Retaining Wall Backfill
Native soils may be used as retaining wall backfill for this project if the total wall height is
4 feet or less and the recommendations below are followed. Native soils for retaining walls
exceeding 4 feet in height must be approved by the local authority or evaluated by an
engineer. Backfill may consist of engineered fill, as presented in this report, or borrow
material approved by a geotechnical engineer. Compaction of these materials shall be
achieved in compacted lifts of about 12 inches. Each lift should be uniformly compacted
to at least 85%, and no more than 90% of the modified Proctor maximum dry density
(ASTM D 1557). If pavement or building loads are planned to be located within retaining
wall backfill, then 90% compaction is required. In addition, heavy construction equipment
should be at a distance of at least ½ the wall height. Over-compaction and limiting heavy
construction equipment should be prevented to minimize the risk of excess lateral earth
pressure on the retaining structure. Envirotech recommends that retaining wall backfill is
compacted with light equipment such as a hand-held power tamper. If clean, coarse gravel
soils are utilized as engineered fill, and surcharges will not influence the retaining wall,
compaction may be achieved by reasonably densifying granular soils with construction
equipment.
5.2.4 Wet Weather Considerations
Due to the types of subsurface soils, additional provisions may be required during
prolonged wet weather. Every precaution should be made in order to prevent free moisture
from saturating the soils within excavations. If the bottom of excavations used for footing
placement changes from a moist and dense/ hard characteristic as presented in this report
to muck or soft, saturated conditions, then these soils become unsuitable for foundation
bearing material. If this situation occurs, a geotechnical engineer should be notified, and
these soils should be completely removed and replaced with compacted engineered fill or
suitable native material as presented in this section.
5.2.5 Building Pads
Building pads for this project, if utilized, shall be constructed per the fill placement and
compaction recommendations as presented above. Both engineered fill and native soils
may be used for building pads. Building pad slopes shall be no steeper than 2:1 for both
compacted engineered fill and re-compacted native soils used as fill. Building pad fill shall
be “keyed” into the existing subgrade to a depth of at least 2 feet below the existing ground
surface. The term “keyed,” as used here, implies that the interface between the building
pad and subgrade is horizontally level. Alternatively, building pads may be keyed into the
subgrade to the above specified depth, and stepped. Stepped fill should be keyed into the
subgrade at a minimum width of 10 feet. All footings shall be located at least 5 feet away
from the top of the engineered fill slope. See the diagram below for a depiction of fill pads
for structural support.
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Envirotech Engineering Geotechnical Report
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Belfair, Washington 98528 Mason County, Washington
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5.3 Building and Footing Setbacks
Provided that assumptions relating to construction occur and recommendations are followed as
presented in this report, the factor of safety for slope stability is sufficient for a 50 feet footing
setback from the face of the nearby descending slopes exceeding 40%. See the figure below and
the Site Plan in Appendix A for an illustration of the setbacks.
From the illustration above, structures may be located closer to the top of slope by extending the
foundation deep enough to maintain the recommended setback. In addition, the required setback
may be reduced by mitigation, and subsequently would require additional geotechnical studies.
Due to potential debris flow, the building location should have a minimum setback from the local
ascending slope toe equal to ½ the slope height. The toe of the ascending slope is delineated as a
grade break in which the ascending slope is in excess of 40%. Envirotech recommends the building
setback to be at least 10 feet from the toe of the nearby ascending slope. See the Site Plan in
Appendix A for an illustration of the setbacks.
5.4 Surface and Subsurface Drainage
Positive drainage should be provided in the final design for all planned residential buildings.
Drainage shall include sloping the ground surface, driveways and sidewalks away from the project
structures. All constructed surface and subsurface drains should be adequately maintained during
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Envirotech Engineering Geotechnical Report
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Belfair, Washington 98528 Mason County, Washington
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the life of the structure. If drainage problems occur during or after construction, additional
engineered water mitigation will be required immediately. This may include a combination of
swales, berms, drain pipes, infiltration facilities, or outlet protection in order to divert water away
from the structures to an appropriate protected discharge area. Leakage of water pipes, both
drainage and supply lines, shall be prevented at all times.
If impervious thresholds are exceeded per the prevailing agency code, then engineered stormwater
management plans are required for this project. The drainage engineer must coordinate with a
geotechnical engineer for input with relation to slope stability prior to submitting drainage plans.
If stormwater management plans are not required for this project, then the following
recommendations should be followed.
For this project, we recommend that infiltration is avoided in order to maintain slope stability, and
that roof downspout dispersion on splash blocks are employed. Recommended drainage details are
provided in Appendix E of this report.
5.5 Vegetation Buffer and Considerations
For this project, we believe that a detailed clearing and grading plan is not warranted unless the
prevailing agency thresholds are exceeded, and basic vegetation management practices should be
adhered to.
Vegetation Buffer – Vegetation shall not be removed from the face of the critical slope or within a
distance of 10 feet beyond the top of the slope, and zero feet from the toe of critical 40% slope.
However, any tree deemed hazardous to life or property shall be removed. If tree removal is
necessary, then stumps and roots shall remain in place, and the underbrush and soil shall remain
undisturbed as much as possible. Any disturbed soil shall be graded and re-compacted in order to
restore the terrain similar to preexisting conditions and drainage patterns. See the Site Plan in
Appendix A of this report for a depiction of the vegetation buffer.
5.6 Temporary and Permanent Erosion Control
Erosion control during construction should include minimizing the removal of vegetation to the
least extent possible. Erosion control measures during construction may include stockpiling cleared
vegetation, silt fencing, intercepting swales, berms, straw bales, plastic cover or other standard
controls. Although other controls may be used, if adequate, silt fencing is presented in this report
as the first choice for temporary erosion control. Any erosion control should be located down-slope
and beyond the limits of construction and clearing of vegetation where surface water is expected to
flow. If the loss of sediments appears to be greater than expected, or erosion control measures are
not functioning as needed, additional measures must be implemented immediately. See Appendix
D for sketches and general notes regarding selected erosion control measures. The Site Plan in
Appendix A depicts the recommended locations for erosion control facilities to be installed as
necessary.
Permanent erosion control is necessary if substantial vegetation has not been established within
disturbed areas upon completion of the project. Temporary erosion control should remain in place
until permanent erosion control has been established. Permanent erosion control may include
promoting the growth of vegetation within the exposed areas by mulching, seeding or an equivalent
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Envirotech Engineering Geotechnical Report
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Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
measure. Selected recommendations for permanent erosion control are provided in Appendix D.
Additional erosion control measures that should be performed include routine maintenance and
replacement, when necessary, of permanent erosion control, vegetation, drainage structures and/or
features.
5.7 Septic Drainfields
Septic drainfields were considered in our geotechnical evaluation. This includes septic drainfields
with relation to the observed soil conditions, expected vegetation removal, and existing and
proposed topography. Based on the aforesaid parameters, the septic drainfields are not expected to
adversely influence critical slopes. This is also based on compliance with all recommendations in
this report, including maintaining septic systems outside of the geotechnical setbacks.
5.8 Structural Mitigation
With respect to landslide alleviation or slope improvements, structural mitigation is not necessary
for this project. This determination is based on the anticipated improvements of the project,
engineering conclusions, and compliance with all recommendations provided in this report.
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Envirotech Engineering Geotechnical Report
PO Box 984 page 20 Parcel 32304-75-90091
Belfair, Washington 98528 Mason County, Washington
Ph. 360-275-9374 August 14, 2023
6.0 CLOSURE
Based on the project information provided by the owner, the proposed development, and site
conditions as presented in this report, it is Envirotech’s opinion that additional geotechnical studies
are not required to further evaluate this project.
Due to the inherent natural variations of the soil stratification and the nature of the geotechnical
subsurface exploration, there is always a possibility that soil conditions encountered during
construction are different than those described in this report. It is not recommended that a qualified
engineer performs a site inspection during earthwork construction unless fill soils will influence
the impending foundation. However, if native, undisturbed subsurface conditions found on-site are
not as presented in this report, then a geotechnical engineer should be consulted.
This report presents geotechnical design guidelines, and is intended only for the owner, or owners’
representative, and location of project described herein. This report should not be used to dictate
construction procedures or relieve the contractor of his responsibility.
Any and all content of this geotechnical report is only valid in conjunction with the compliance of
all recommendations provided in this report. Semantics throughout this report such as ‘shall,’
‘should’ and ‘recommended’ imply that the correlating design and/or specifications must be
adhered to in order to potentially protect life and/ or property. Semantics such as ‘suggested’ or
‘optional’ refer that the associated design or specification may or may not be performed, but is
provided for optimal performance. The recommendations provided in this report are valid for the
proposed development at the issuance date of this report. Changes to the site other than the expected
development, changes to neighboring properties, changes to ordinances or regulatory codes, or
broadening of accepted geotechnical standards may affect the long-term conclusions and
recommendations of this report.
The services described in this report were prepared under the responsible charge of Michael Staten,
a professional engineer with Envirotech. Michael Staten has appropriate education and experience
in the field of geotechnical engineering in order to assess landslide hazards, earthquake hazards,
and general soil mechanics.
Please contact Michael Staten at 360-275-9374 if you have any questions, comments, or require
additional information.
Sincerely,
Envirotech Engineering
Jessica Smith, M.S. Michael Staten, P.E.
Staff Geologist Geotechnical Engineer
APPENDIX A
SITE PLAN
APPENDIX B
SOIL INFORMATION
APPENDIX C
SLOPE STABILITY
APPENDIX D
EROSION CONTROL
APPENDIX E
DRAINAGE DETAILS